Vented fireplaces come in three vent types: conventional, power and direct.
Conventional vents are those seen everyday, where exhaust is vented through a chimney using convection.
Power vents exhaust flue products using a power driven blower. They are particularly useful on horizontal or off-vertical runs where it is necessary to supplement the natural bouyancy of the flue products. The decrease in lift from the exhaust on a horizontal run can make it difficult to vent products of combustion.
A direct vent fireplace uses a two duct system. Combustion air is drawn from the outside of a dwelling through one duct, while the exhaust gases are vented through the other duct. This overcomes much of the problem of drawing air for combustion from within the house and allows for vertical and horizontal runs within limits specific to the fireplace specifications without the use of a fan.
In most direct vent units available today, the ducts come in the form of a single double-walled co-axial duct. The exhaust flows through the inner duct and the combustion air in the outer. The exhaust heats the combustion air increasing the efficiency of the unit and the combustion air cools the exhaust air before it is released outside, allowing the vent to be placed nearer to objects which could be affected by heat.
Co-axial direct vent units come in one of two forms, top and rear vent. Rear vent units vent exhaust from an opening in the rear of a firebox through a concentric opening at the rear of the unit, while top vent units vent from an opening on the top of the firebox through a concentric opening on the top of the unit.
In order to have the duct of a rear vent unit exit a building at a point other than directly behind the opening at the rear of the unit, the unit must be placed away from the wall and the duct must have a bend or elbow in it. Unless the desired exit is in direct line with the bent duct, this must be followed by a further bend. This results in a unit which is further away from the wall than may be desired and a duct which takes some effort to install.
The opening in a top vented unit is over the firebox and, as a result, is only a short distance from the front of the unit. Usually this distance is insufficient for a mantlepiece, unless the duct is bent towards the rear of the unit. Even so, the mantlepiece must be placed at a height above the forward portion of the duct. This can detract from the appearance of the finished fireplace. Also, even if the fireplace can be vented to the rear at the height of the fireplace, a full elbow is required to direct the duct.
Most modern gas fireplaces are pre-fabricated units. The basic types are zero clearance, insert, free-standing and wallmount. A zero clearance fireplace, as its name suggests, can be placed against combustible materials. An insert is a unit made to fit within an existing solid fuel burning fireplace, and may use the existing chimney. A free-standing unit may be placed on the floor in a room without support from the wall and may be spaced a distance from the wall. A wall mount is placed on and supported by a wall in the room.
Air flows by convection or with the use of a fan, through a passage between the bottom of the firebox and the bottom of the unit, around the rear and sides of the firebox, and out across the top of the firebox directing heat into the room. In order to increase the efficiency of the unit, some manufacturers place a heat exchanger for top vented units in-line with the exhaust outlet over the top of the firebox in the circulating air passage. The heat exchanger impedes the flow of the exhaust gases and provides additional heat transfer surface area which allows for additional heating of the room air.
In many jurisdictions, the firebox must contain relief doors to relieve pressure which may be created from a gas explosion. An explosion can occur when an explosive mixture of gas and air accumulates within the firebox and is ignited. The maximum pressure under these conditions must be kept below the pressure that would break the viewing window or otherwise damage the fireplace. The preferred position for relief doors is at the top of the firebox. This allows for gravity closure of the relief door, simplifying the design.
As space is limited in direct vent gas fireplaces some manufacturers have decreased the size of the relief door on top vented units and added a relief door at the rear of the firebox.
Typically, in direct vented units the combustion air is brought down the rear of the unit and enters the rear of the firebox through a pair of tunnels. In order to prevent the combustion air from unnaturally impinging on the flame, canopies are placed over the tunnel entrances to the firebox in an effort to force the air to come from below the burner. To improve on this, manufacturers have used a false bottom over the canopies which better directs the air flow to the burners. Neither of these solutions are optimal.